Flexible cast-coated paper

ABSTRACT

A CAST-COATED PAPER IS DISCLOSED IN WHICH THE COATED PAPER HAS IMPROVED PROPERTIES OF FLEX, PLY, STRETCH, AND A MORE NATURAL LIMP, RAG-LIKE FEEL. THE IMPROVED PROPERTIES ARE ATTAINED BY SELECTING FOR THE SUBSTRTE CERTAIN SATURATED PAPERS HAVING POROSITIES LYING IN A DEFINED NARROW RANGE.

United States Patent Oce 3,681,116. Patented Aug. 1, 1972 3,681,116 FLEXIBLE CAST-COATED PAPER Eugene H. Reich, Kenhorst, and Frank J. Zasztowt, Reading, Pa., assignors to Wyomissiug Corporation No Drawing. Filed Apr. 29, 1970, Ser. No. 33,092 Int. Cl. B4411 1/44 US. Cl. 117-64 C 2 Claims ABSTRACT OF THE DISCLOSURE A cast-coated paper is disclosed in which the coated paper has improved properties of flex, ply, stretch, and a more natural limp, rag-like feel. The improved properties are attained by selecting for the substrate certain saturated papers having porosities lying in a defined narrow range.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to cast-coated papers and, more particularly, to cast-coated papers that have improved properties of flex, ply, stretch, and a more natural limp, rag-like hand.

(2) Description of the prior art The cast coating of paper substrates was originally disclosed in US. Pat. 1,719,166. As cast coating is generally practiced today, an aqueous coating mixture consisting of pigment and an adhesive, such as casein, is applied to one side of a paper web, and the coated surface is then pressed against a heated, highly polished finishing surface, such as a chromium-plated drum, and the coating is dried while in contact with this drum. When the paper is released from the surface of the drum, it will have a finish that, to the unaided eye, is substantially a mirror image of the metal finishing surface. Casting is considered to be the process of hardening the pigmented coating surface against a polished metal drying drum to produce a surface corresponding to that of the drying drum. The drying drum surface is called the casting surface.

Cast-coated papers are quite desirable for some uses due to their high coating weights, their high bulk and greater ink absorption as compared with papers made by conventional processes involving supercalendering, which tends to compress and density the coating, and, most importantly, the coated papers are desirable for their ultra high finish and gloss. Partly due to the high coating weights that are applied to cast-coated papers, these papers are relatively stifl, which stifiness may be objectionable in some applications. Further, when castcoated papers are bent to a sharp angle, the coating will usually fracture and expose the paper substrate. Since many applications for high gloss cast-coated papers are for decorative purposes, such as wrapping papers and the like, the comparative inflexibility of the coating and its tendency to crack away from the paper when folded is particularly undesirable.

It has now been found that cast-coated papers can be manufactured which, when compared to those known to the prior art, have a much higher degree of flex, ply and stretch, while at the same time they preserve all of the desired properties of the cast-coated paper, such as high gloss.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to prepare a cast-coated paper that has improved properties of flex, ply and stretch.

Another object of this invention is to provide a castcoated paper in which the coating will not crack or fracture when the paper is repeatedly flexed or creased at a sharp angle.

Another object of this invention is to prepare a cast- ;oalted paper that has a comparatively limp and rag-like Briefly, these and other objects of this invention are attained by selecting as the paper substrate a saturated paper having a porosity within a narrowly selected range.

By the use of such saturated papers, the coating, quite surprisingly, displays a much lesser tendency to crack and is considerably more flexible than the coatings applied to ordinary paper substrates that are not saturated.

DETAILED DESCRIPTION OF THE INVENTION Saturated papers such as are of utility for use as a substrate in this invention are well known in the industry. Generally, they may be defined as papers that have been impregnated with various synthetic and natural thermoplastic resins or rubbers in order to improve the physical and/or optical properties of the paper. It should be understood that the term saturated refers to the fact that the paper contains some resinous component and not to the fact that the papers are saturated with some material. In point of fact, the quantity of resins that may be added to the paper can vary over a considerable range. For most applications, the amount of resins added to the paper are somewhat less than would be required to close all of the pores of the paper and cause the resin to exist in a continuous phase. If resins are added to this extent, the absorptive properties of the paper will be lost and they will not readily receive inks.

In the cast coating process, the water of the coating composition is removed from the wet coating variously by being absorbed into the paper substrate and by evaporation through the back surface (the side away from the drum) of the paper. The time that the coated surface of the paper is in contact with the heated drum is of considerable importance. On the one hand, the time must be of sufficient duration to enable removal of substantially all of the water in order to form a hard surface coating, and, on the other hand, the time must not be too great, for, if it is, the coating surface may lose its gloss and begin to yellow. As the primary removal of water takes place by evaporation, the rate at which water can be removed from the coating material is, to some extent, a function of the porosity of the paper. With many papers, the porosity can vary over a considerable extent, but, however, in the case of saturated papers, a porosity must be selected within a comparatively narrow range if successful cast coating is to be achieved.

The reason for the greater criticality of the porosity of the saturated papers in cast coating operations lies in the fact that the resinous content of the paper changes the capillary structure of the paper. As the resins will tend to coat individual fibers of the paper, internal absorption of the water component of the aqueous coating is materially decreased. Also, the total number of open pores in the paper is reduced as some are filled by the resins. It is believed that in most instances, saturated papers have a considerably less total number of pores, although the pores that do remain are comparatively open and pass relatively straight through the paper as opposed to a relatively more tortuous path of the pores in papers not containing resin.

For purposes of this specification and appended claims, the porosity of the saturated papers is given as determined on a Gurley Porosity Densitometer. In this test, a piece of paper is sealed over a standard orifice and the time in seconds is recorded that is required to pass a given volume of gas at a given pressure through the paper. Accordingly, the porosity will be referred to in units of seconds and a low number indicates a high porosity and a high number indicates a comparatively low porosity.

The resins with which the paper may be saturated may vary considerably, but the following are listed as examples of more commonly used ones readily available on the market. All of them are sold by B. F. Goodrich Chemical Company under the trade designations listed.

Hycar-These are defined as being lateXes of butadieneacrylonitrile or polyacrylics.

Good-RiteThis is a butadiene-styrene latex.

Geon- Fhis may be a vinyl chloride or homopolymers and copolymers of vinylidene chloride, vinyl chloride or acrylics.

While the cast coating techniques used in accordance with this invention are not critical, in the preferred practice of this invention, techniques disclosed in US. patent application Ser. No. 721,694, filed Apr. 16, 1968, now Pat. No. 3,600,215, are preferred. This application generally describes an improved cast coating method in which an aqueous mineral cast coating composition having a pH above and below 7 is applied to a traveling web of paper. The coated paper web is moved in an upward direction and, while so moving upward and supported on its uncoated side, a uniform jet stream of air is directed onto the Wet coating. The air stream has a width substantially equal to that of the coated paper web and is directed in a direction toward the coating but opposite to the direction of travel of the Web. After the web has been so coated, the coated side is pressed into nonslipping, adherent contact with the traveling, heated, polished casting surface, and the coating is dried to a substantially nonplastic condition while in contact with the casting surface.

The advantage of using this particular technique lies in the fact that certain improved results are obtained such as lack of curl in the paper, a high quality gloss coating free from pits and other flaws, and appreciably higher speeds of operation. The exact reason for these results is not known, but it is believed that they are a combination of the coating composition having the slightly acid pH (as contrasted to the alkaline compositions more commonly used) and to the jet stream that is applied in the manner as described.

As detailed in the referenced patent application and other literature teachings relating to cast coating, a typical cast coating composition will include a binder, which may be a synthetic latex or a hydrophyllic protein adhesive, such as casein or soya bean protein in an aqueous vehicle, and a mineral filler, such as a fine, coating-grade clay, calcium carbonate, blanc fixe, satin white, titanium pigments, color pigments, and the like, and various combinations thereof. Formaldehyde (including the conventional formaldehyde donors, like monomethylol dimethyl hydantoin) may also be included in the coating composition to render the protein in the final coating relatively water-resistant. In these respects, the present coating composition may be similar to those used in the past, and in which the preferred binder is casein and the preferred filler is clay, with colored pigments when desired. Likewise, the composition may, in accordance with conventional practice, also contain a release agent which aids in separating the coated web from the casting drum surface, such as a fatty acid soap, like ammonium stearate, zinc stearate, and the like. The composition may also contain auxiliary or supplemental binders like acrylic resin, butadiene-styrene latex, and the like, and a protein viscosity-reducing agent, like dicyandiamide.

The coating composition of the present invention, however, will have a weakly acid pH; that is, a pH above 5 and below 7, preferably between 5.4 and 6.6. This can be provided through the use of a weak organic acid, such as formic, acetic, citric, and the like. Since the protein is normally initially solubilized by treatment with alkali, preferred practice involves markedly reducing the amount of alkali normally used in solubilizing the protein and adding sufficient weak acid subsequently to adjust the pH to within the stated range. Commercially aqueous formaldehyde solution (Formalin) is acid, containing formic acid, and may be relied upon to provide the desired pH. On the other hand, a weak acid as such may be used in place of or in conjunction with the Formalin to adjust the pH. When formaldehyde is omitted from the coating composition, the cast coated paper may subsequently be treated with a water proofing agent, such as a formaldehyde, to provide enhanced water-resistance, if desired. Of course, water-resistance in the coating may be provided by means other than the use of formaldehyde, as by the use of water-resistant auxiliary or supplemental binders of the type mentioned above.

In the coating composition, the content of solids (binder and pigment, and release agent and/or viscosity-reducing agent, when used) will generally be between about 45 and about 60%, by weight, based on the total weight of the composition. As between the binder and pigment, the former will normally be present in a minor proportion, that is, less than 50%, usually in an amount between about 10 and about 30% by weight; and the pigment will normally be present in a major proportion, that is, more than 50%, usually in an amount between about 70 and about Formaldehyde (as 37% formaldehyde solu tion) may be present in an amount between about 1.5 and about 21% by weight, based on the dry weight of the protein binder.

As mentioned above, it has been found of great importance to select a saturated paper having a porosity in a comparatively narrow range. If the porosity is too great, moisture will be removed from the paper too rapidly and the time the paper is in contact with the heated drum will not be sufiicient to form a highly polished surface on the paper. On the other hand, if the porosity is too low, the paper will have to remain in contact with the heated drum for a comparatively long period of time to dry it which may result in loss of gloss and some yellowing. It has been determined that satisfactory results can be obtained using saturated papers having a porosity between about 4 and 30 seconds and, more preferably, using saturated papers having a porosity in range of about 5 to 15 seconds.

EXAMPLE I A saturated paper (obtained from Kimberly Clark) was selected for use in this experiment that was saturated with an acrylic latex. The paper was a modified kraft paper and the acrylic latex was applied as an impregnant after manufacture of the paper. (It should be noted that resins can also be beater added in making saturated papers.) The paper had a weight basis of 28 pounds per ream (a ream being 500 sheets-24 x 20") and had a porosity in the range of about 8 to 9 seconds. The following coating composition was prepared. (Parts are by weight.)

The solids content of this composition is about 52%, its pH is about 5.7-6, and its viscosity, is between about 250 and about 1000 cps. Brookfield at room temperature using a No. 3 spindle at 12 rpm.

Sheets of the saturated paper were then cast-coated using the techniques described above and in the referenced patent to the common assignee. The paper so cast-coated had an exceptionally high gloss, was quite flexible and had much of the hand and drape of the uncoated saturated paper.

As a control, a kraft stock having a weight basis of 22 pounds per ream and comprised of 20% broke, was cast-coated with the same formulation. There was a material observable difference between the control and the saturated paper in that the control was less flexible and comparatively stiff.

Both the saturated paper and the control were subjected to a fold test. The fold test was performed by folding the paper away from the coating and against the grain of the paper through an arc of 180. A 2 pound weight (2" x 3" metal cylinder) was then rolled over the fold of the paper once. The paper was unfolded and held against a source of light. This permits the amount of cracking to be determined by visual examination.

In this example the saturated cast-coated paper was totally free from any visible cracks whereas at least 50% of the coating along the line of the fold was cracked away in the case of the control.

EXAMPLE H In this example a saturated paper similar to the one used in Example I was coated. The paper had a porosity of about 5 seconds and a weight basis of 26 pounds per ream. The coating formulation used was as follows:

This composition has a pH of about 6. The addition thereto of 5.5 parts of 37% formaldehyde lowers the pH to 5.1-5.3. In this composition, formic acid or citric acid may be substituted for the acetic acid, solubilized soya bean protein may be substituted for the casein, and calcium carbonate or other equivalent mineral pigment may be substituted at least in part for the clay.

A control was also coated using this formulation and the results were substantially identical to those reported in Example I.

We claim:

1. A method of making a cast-coated paper which is flexible and can withstand creasing without cracking the coating, the method comprising coating the surface of a resin-impregnated paper substrate having a Gurley pm rosity in the range of between 4 and 30 seconds with an aqueous mineral cast-coating composition and drying the coating while in contact with a polished heated drum.

2. A method according to claim 1 wherein the porosity is in a range of between 5 and 15 seconds.

References Cited UNITED STATES PATENTS Re. 25,039 9/1961 Frost et al 117-64 C 2,173,097 9/ 1939 Coggeshall 117--83 2,263,575 11/1941 Glenn 11783 2,678,890 5/ 1954 Leighton 11765.2 X 3,066,043 11/ 1962 Hechtman et al. 117-76 P X 3,156,581 11/1964 Finnegan et al. 117-76 P 2,810,966 10/ 1957 'Bicknell 117--64 R X 3,413,139 11/1968 Rasmussen et al. 117-83 X 3,338,734 8/1967 Golf et al 11765.2 X 2,188,331 1/ 1940 Coggeshall 117-76 P WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner U.S. Cl. X.R. 

